Role of Sulfite Oxidation in NO2Absorption in a pH-Neutral Scrubber Solution

Abstract

Nitrogen dioxide (NO2) absorption in aqueous sulfite solutions has recently gained more attention to reduce NOxemissions. The NO2removal efficiency is strongly influenced by the competing reactions of NO2absorption and sulfite oxidation. The present study investigates the role of sulfite oxidation in NO2absorption at neutral pH. The effects of sulfite concentration, thiosulfate, pH, NO2inlet concentration, oxygen concentration on NO2absorption efficiency, and sulfite oxidation rates were investigated under well-controlled conditions at 25 °C in a laboratory-scale wet scrubbing system. NO2absorption rates showed a strong dependence on the sulfite concentration and increased linearly with increasing NO2inlet concentrations between 25 and 150 ppm. At the highest investigated sulfite concentration, 20 mM, the NO2removal efficiency was around 80%. Oxidation rates of sulfite during NO2absorption in the presence of 5 vol % O2increased with increasing sulfite concentrations in the scrubbing solution and ranged from 20 mg·L-1·min-1with 1 mM sulfite to 200 mg·L-1·min-1with 20 mM sulfite. The oxidation rate decreased significantly when thiosulfate was added, while the oxidation rates were independent of the oxygen concentration between 2-10 vol % O2. The NO2absorption rate decreased with decreasing pH between pH 6 and 8 proportionally to the sulfite concentration as predicted by the sulfite-bisulfite equilibrium. The implication from these results is that significantly less sulfite feeding is needed in industrial NO2absorption if the pH can be maintained at neutral pH, instead of lower pH such as in simultaneous SO2and NO2scrubbing.